Four Arab men were held here today after last night's terrorist shooting of Israeli Ambassador Shlomo Argov, who remained in critical condition after an operation to remove part of his damaged brain where a single bullet had passed completely through his head.

Police sources said they had reason to believe two of the arrested men are from Jordan, one is from Iraq and one from Syria. The sources added that they suspect some of them may be Palestinians residing in those countries. Only one of the suspects had a passport, according to police sources, and it is thought to be false. Police said the men would be named when they are charged in court.

A senior police official said tonight that the arrests of the men may have prevented other terrorist attacks here. "We deplore this cruel crime," said Deputy Assistant Commissioner David Powis at Scotland Yard. "But we believe we have frustrated a series of terrorist outrages."

After Argov, 52, was shot in the head while leaving a banquet at the Dorchester Hotel in central London, his fleeing assailant was shot in the neck and arrested in a side street around the corner. Police said the gunman, being guarded in a London hospital, is expected to recover from his wound.

Two other men were arrested early this morning in south London in what was believed to be the assailant's getaway car. A fourth man, believed to be the head of the assassination team, was arrested later today in a rural area south of London.

Police said they seized a Polish-made 9-mm submachine gun, two Beretta handguns, a large amount of ammunition and hand grenades. The submachine gun, believed to be the shooting weapon, was found near where Argov's assailant was shot and arrested.

A spokesman for the National Hospital for Nervous Diseases here, which has among its specialties neurosurgery, said Argov's condition had not deteriorated after an overnight operation to relieve dangerous swelling in his brain by removing clotted blood and damaged tissue. The spokesman said the bullet entered the right side of Argov's skull, passed completely through his head and came out the other side.

"We must emphasize that the ambassador is still critically ill and at this stage we cannot predict what the outcome may be," the hospital spokesman said. Other sources said Argov was still unconscious today.

No terrorist group has yet claimed responsibility for the assassination attempt. The London representative of the Palestine Liberation Organization, Nabil Ramlawi, denied PLO responsibility and said the shooting "served Israeli and not Palestinian interests."

Referring to earlier attacks on Israeli and American diplomats in Europe for which a previously unknown Beirut-based terrorist group has claimed responsibility, Ramlawi said, "The Israeli ambassador is a victim of a determined campaign now being waged in European capitals to discredit the PLO and to undermine the understanding and friendship which exists between the Palestinians and the [European Common Market] governments."

Ramlawi has been a key figure in a PLO campaign to gain support in Europe through diplomatic initiatives to which the Common Market countries have been cautiously receptive. Argov and other Israeli diplomats in Europe have publicly complained that too much attention has recently been given to the Palestinians.

Both British Prime Minister Margaret Thatcher and Foreign Minister Francis Pym have sent messages of sympathy to Israeli leaders. The Foreign Office here called the shooting "an outrageous act against a most respected member of the diplomatic community."